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Friday, 24 October 2014

Can we make room temperature superconductors?

What do high speed levitating trains, MRI machines and particle accelerators have in common? They all use superconductors. Superconductors are materials that can carry electrical current for long distances without losing energy, and can even produce their own magnetic fields.

Why are superconductors important?

These materials have a vast and diverse range of uses, mainly because they allow for the production of extremely efficient wires. The relationship between electric current in wires and magnetic fields is an intimate one - a magnetic field is created every time an electric charge moves, and every time a magnetic field is changed an electric field is created. This means superconducting materials play an important role in creating efficient and powerful electromagnets. These can be used to construct MagLev trains that float above the tracks, eliminating friction and allowing them to travel at incredibly high speeds, in MRI scanners, and even in particle accelerators such as the Large Hadron Collider where the Higgs Boson was discovered! This is an incredibly exciting prospect - not losing power to electrical resistance could have a profound effect on saving energy resources.

The Shanghai Transrapid maglev train has a top speed of 431 km/h (268 mph), racing the 30km from Pudong International Airport to downtown Shanghai in just 7 minutes and 20 seconds. Image credit: Lars Plougmann

How do superconductors work?

Materials that conduct electricity do so because they contain electrons that can move, and this is the same in a superconductor. Electrons in a normal conductor travel alone and sometimes bump into each other, losing energy, which contributes to the resistance of the material. However, electrons in a superconductor are paired up (this is called a “Cooper pair”) and so they move through the structure much more smoothly, leading to a lower resistance.

The electrons in a conductor travel through a lattice of positively charged ions. Because the electron is negatively charged, it is attracted to each ion in the lattice - a force which is stronger as the electron gets closer. This is the same as magnets where opposite poles attract each other and like poles repel. As the first electron passes through the lattice, the ions are pulled towards the electron, distorting it. The ions don’t close up completely though, as they repel each other, creating a tunnel of positive ions. This tunnel attracts the second electron, and so it easily travels through exactly the same path as the first electron. Essentially, the tunnel of ions created acts as a miniature accelerator, so the second electron travels faster as it is pulled towards the positive charge. The electrons pair up because the repulsion that two electrons would normally experience, because they are both negatively charged, is partially overcome by the positive charge of the ions pulling the second electron towards the first - but other electrons are pushed away by the combined fields of the first two. If these pairs are stable, the material becomes superconducting.

Electrons (blue) passing through a lattice of positive ions (red) distort the lattice towards themselves, and form into "Cooper pairs" as a result. Image credit: Things We Don't Know (CC BY 3.0)

What are the limitations?

Unfortunately there’s a catch. In order to become superconductive, most materials have to be cooled below their often incredibly cold critical temperature - the name given to the temperature above which the material ceases to be superconducting. This often involves cooling with liquid nitrogen, which creates a mammoth logistical and safety problem, as liquid nitrogen is often used at -196°C! In fact, most superconductors will only work at a temperature below a rather chilly -110°C. This is because Cooper pairs are easily split up by heat energy, and therefore it is only at very low temperatures that there are enough pairs of electrons present for the material to become superconducting. So how do we create a superconductor that works at room temperature? That’s the big question!

How could room temperature superconductors work in the future?

In 2013, US scientists proposed a method for making superconductors that could operate at higher temperatures using metamaterials[1]. These are artificial materials that have properties different to those of the constituent materials because they gain their properties from the structure of the atoms within the material, rather than the type of atoms. This involved making a new material from a conventional low temperature superconductor and a dielectric material in order to create a material with a higher critical temperature. A dielectric material is an insulator which can be polarized (the charge separated) by applying an electric field. The proposal drew inspiration from Russian physicist David Kirzhnits’ description of a superconductor in 1973[2], which states that the weaker the dielectric response of a superconductor, the stronger the interaction between electrons (and vice versa).

This proposed room temperature superconductor therefore draws on the fundamentals of how superconductors work. Remember that Cooper pairs break down at higher temperatures, which explains why materials can only become superconductors when they are extremely cold. From Kirzhnit’s description of superconductivity, we can predict that using a material with a small dielectric response would make a good room temperature superconductor. This is because there would be a stronger interaction between electron pairs, and so they are less likely to fall apart when the superconductor is warmed up. This is exactly what Vera Smolyaninova of Towson University and Igor Smolyaninov of the University of Maryland hypothesized in 2013. Experimental tests on this type of material are being carried out and it will be very interesting to see if the mystery of room temperature superconductivity will be solved by this prediction in the near future.

The most recent breakthrough in superconductor science was made in June 2014, although the research is still in its early stages[3]. Rather than directly trying to make a new superconducting material, researchers at the University of Cambridge, UK identified the origin of superconductivity in so-called high temperature superconductors.

“High temperature superconductor” is somewhat of a misnomer, because they actually work at -135°C, but this is certainly a large step forward from the materials that have to be cooled almost to absolute zero.

The key to this discovery was to truly understand what the “glue” is that holds the electrons in Cooper pairs in superconducting materials, by breaking the electron pairs apart. The strength of a superconductor increases as the strength of the “glue” increases. The researchers applied a very strong magnetic field to cuprates (a lattice containing copper and oxygen atoms, as well as other elements) and were able to stop them from superconducting. This is because when the superconductor is exposed to a high magnetic field strength, the attraction between the electrons in a pair is weakened and so the Cooper pair breaks apart. It actually took a magnetic field of 100 Tesla (that’s 1 million times stronger than the Earth’s magnetic field!) to kill the superconducting properties of the cuprates! They found that charge density waves (which are essentially “ripples” of electrons) create pockets of electrons in certain materials. These experiments showed that there is in fact a “twisted pocket geometry” of electrons. This means that each layer lays in the opposite direction to the ones above and beneath it, a bit like Jenga blocks. This particular geometry is the source of the electrons that pair up when the material becomes a superconductor. The results of this study showed the locations in the material where superconductivity is at its weakest. Their origin actually lies in the charge density waves. In the case of the cuprates tested, it is this “normal state” that is overridden when electrons pair up to form Cooper pairs, resulting in superconductivity. The next step would be to find materials with similar properties and therefore identify superconductors that can operate at higher and higher temperatures - which could eventually lead to a fully functioning room temperature superconductor.

It's clear that plenty of further research is required before we can say that we have found a room temperature superconductor, but we are tantalizingly close!